According to the police report, when an officer showed up to the house and asked Gates to step outside so he could discuss a potential break-in in progress, Gates asked, "Why, because I'm black in America?" (He's a prolific author of books on the African-American experience.)
Although the police officer believed that Gates was in his home lawfully (Gates produced his ID card), he claimed he was surprised and confused by Gates' behavior toward him. After Gates exhibited "loud and tumultuous behavior" by continuing to yell, the officer handcuffed him, put him under arrest, and took him in a police cruiser to be booked.
Some in the community are wondering: would this have happened to a white professor? Whether or not the motivation was racial, it sure seems like an overreaction to arrest a person who's at his own house for "loud and tumultuous behavior."